Governments need to be able to communicate on a worldwide basis for diplomatic, economic and development reasons among others. Without taking precautions, it could be possible to eavesdrop on these communications by detecting even miniscule electromagnetic energy emitted by communications apparatus.
In order to be safe from increasingly sophisticated electronic eavesdropping techniques, governments typically require that their offices conduct sensitive communications from inside a special shielded enclosure that eliminates or, at least, drastically minimizes (i.e., attenuates) electromagnetic leakage especially at radio-frequencies (RF). The extent of attenuation from RF emissions within an enclosure is the subject of a variety of technical standards. As an example, in typical U.S. government specifications, e.g., NSA 65-6/94-106, the RF attenuation necessary to secure these communications is at least 100 dB at a frequency of 10 GHz.
Because the complexity of secure communications from enclosures worldwide varies considerably, the enclosures themselves vary in size from small chambers to entire suites of electromagnetically secure rooms. The doors to these enclosures are perhaps the most vulnerable to leakage of RF—RF emissions that escape the enclosure. Current doors are designed with heavy-duty metal shielding and heavy latches that attempt to seal the enclosure from RF leaks. These doors are heavy and, therefore, expensive to ship around the world. It is a challenge to handle them without damage, and they are hard to install. The conventional means for allowing such doors to open and close repeatedly while still maintaining a seal against RF leakage when the door is closed is by using so-called “finger stock”. Typically manufactured from copper, finger stock is installed around a door frame. A corresponding knife-edge, often brass, is installed on the door's perimeter. Both the finger stock and the knife-edge are exposed when the door is opened, making them susceptible to accidental deformation or injury from persons or equipment entering or exiting the RF shielded enclosure. When the door is closed, a heavy-duty latch causes the knife edge perimeter to contact and pressure the finger stock to form a RF seal. The door's inner edges must fit precisely against the finger stock frame. If, however, the finger stock becomes distorted, bent or out of shape, as often happens, or if the knife edge is bent or deformed then imperfections are created which allow RF emissions to leak out of the enclosure. At microwave frequencies, even extremely small holes, defects or other imperfections can be a source of an RF leak.
An example of a prior art door is shown in FIG. 1. A close up view of finger stock 102 on a RF door 103 is shown in a closed position. The finger stock 102 is installed around the edge perimeter 104 of the door frame 101. A corresponding knife-edge is installed on the door's edge perimeter 104. To provide complete sealing against RF leakage, the finger stock 102 must exist around the entire perimeter of the door frame. When the door is closed, a heavy-duty latch (not shown) causes the knife edge perimeter 104 on the door 103 to contact and pressure the finger stock 102 of the door frame 101 thereby forming an RF seal. An RF seal is said to exist for an enclosure when RF transmissions outside the enclosure, which the door 103 is a part of, do not penetrate into the enclosure and RF transmissions generated within the enclosure do not escape. In both instances any leakage of RF transmissions into, or out of, the enclosure can be measured by applicable technical standards. Continuing, with the door 103 shown in FIG. 1, the knife edge 104 may be placed in many orientations relative to the finger stock 102. Over time, after the door 103 is opened and closed a number of times, finger stock 102 becomes worn and needs to be replaced.
Doors to larger or busier facilities may be opened and closed many times, inevitably causing increased wear and tear at the door edges. Such wear and tear often causes the enclosure to no longer meet applicable RF attenuation specifications. Realizing the problems associated with such wear and tear, users of existing doors must maintain their doors periodically so that the doors will continue to function in accord with applicable technical standards. Among other maintenance requirements, finger stock must be cleaned often and replaced at least every six months by a skilled tradesman.
The finger-stock used to help reduce RF leakage must exist around the entire perimeter of the door frame, including the floor, to provide adequate RF signal attenuation. For this reason, the door frames used in existing RF shielded enclosures have high, raised structural thresholds necessary to hold the finger stock. Accordingly, current RF shielded doors using finger stock will not meet the access requirements of the Americans with Disabilities Act of 1990, popularly known as the ADA. See Pub.L. 101-336, 104 Stat. 327 (Jul. 26, 1990) codified at 42 U.S.C. §12101 et seq.
The problems outlined above suggest there is a strong need to minimize RF leakage from a shielded enclosure without using finger stock while ensuring that applicable RF attenuation standards and access requirements of the ADA are met or exceeded.